Priority boarding has been a GR benefit since at least 2000 (when I rejoined MPC, having been a GR/SL in the mid-1990s).
One embarrassing incident with being picked to board ahead of others - I was on HKG-LHR in F, I was picked and asked to stand at the "third line". I was the only one there. People were staring at me. One man even told me to get back into the priority line, thinking I was cutting the line. The gate staff had to reply to the man "this customer is in First Class".... I was so embarrassed!